The Twins, normal mode and expert mode. Like, I don't really get at all why everyone seems to find them so difficult. I guess there are two bosses attacking you at once so that could be kind of difficult to deal with, but all of their attacks are really easy to dodge, even when put together. I honestly had a harder time with the Destroyer in my first playthrough.
I guess another, although to a lesser extent, would be Plantera. Plantera's the same kind of case where just dodging makes the whole fight a lot easier, but I there's also the additional difficulty of having to fight underground so I won't say it's a really easy fight. Plus, Plantera gets a lot more difficult in expert mode.